$\dfrac{8}{4} - \dfrac{9}{10} = {?}$
$ = {\dfrac{8 \times 5}{4 \times 5}} - {\dfrac{9 \times 2}{10 \times 2}} $ $ = {\dfrac{40}{20}} - {\dfrac{18}{20}} $ $ = \dfrac{{40} - {18}}{20} $ $ = \dfrac{22}{20}$